Process of preparing tanning material and by-products derived therefrom



Patented Nov. 7, 1939 UNITED STATES PATENT OFFICE I PROCESS OF PREPARING TANNING MA- TERIAL AND BY-PRODUCTS DERIVED THEREFROM No Drawing. Application May 6, 1937, Serial No. 141,127

20 Claims.

This invention relates to a process for reducing hexavalent chromium, and more particularly to a process for preparing a tanning material and useful by-products derived therefrom.

In the recovery of chromium from chromite and the like ores it is the usual practice to roast the ore under oxidizing conditions with a suitable alkali and subject the roasted product to a leaching treatment to extract the water-soluble chromium compounds. .The chromium thus recovered is invariably in the hexavalent state and consequently unsuitable for use as a tanning material by the modern one bath process of chrome tanning of leather. It is therefore necessary to reduce the hexavalent chromium to the trivalent state, and to this end it is the practice to treat an aqueous hexavalent chromium solution with glucose or other reducing agent in the presence of sulphuric acid. This treatment is considered wasteful in that the oxidizing power of the hexavalent chromium is not employed for any useful purpose, the glucose being oxidized largely to carbon dioxide and water.

The principal objects of the invention are to overcome the aforementioned objectionable feature of the prior practice and to provide an economical and efiicient process for reducing hexavalent chromium to the trivalent state and simultaneously produce by-products of recognized commercial utility.

Further objects will be apparent from a consideration of the following description which discloses different embodiments of the invention chosen for illustration.

In accordance with the present invention, a hexavalent chromium compound, such for example as a dichromate derived from a leached chromite roast, is first converted into a chromic acid, preferably by treatment with sulphuric acid, and the resulting product is then treated with an unsaturated organic compound under conditions which are effective to reduce the hexavalent chromium to'the trivalent state. The selection of the particular type of unsaturated organic compound, apart from commercial considerations, depends upon the particular by-products sought to be produced, the ratio of the reactants, temperature and such other factors as may vary the trend of the reaction. Where, as is preferred, the unsaturated organic compound consists essentially of one or more unsaturated fatty acids, the reaction proceeds as follows:

where R may be hydrogen, a hydrocarbon radical, or a substituted hydrocarbon radical, and a: may be zero or any whole number. This equation shows that by the oxidation of the organic com- Oleic acid Pelargonic acid COOH(CH2)1COOH Azelaic acid In the case of petrosel'ic'acid, an isomer of oleic acid, in which R is the hydrocarbon. radical, CH3(CH2)10-, and w is 4, oxidation results in the formation of lauric acid (dodecanoic acid) and adipic acid. In the case of ricinoleic acid 12- hydroxyoctadecen-Q-oic acid), in whichR is the substituted hydrocarbon radical, CH3(CH2)5 CHOHCH2, and a: is '7, oxidation results in. the formation of 3-hydroxypelargonic acid and azelaic acid. As is shown hereinafter for the case of linoleic acid, where R consists of a hydrocarbon radical containing a point of unsaturation, this behaves in the same manner as the point of unsaturation shown in the general equation above when oxidized by the. method I have proposed.

Although any unsaturated fatty acidmay be used to reduce the hexavalent chromium, some consideration must be givento the location of the points of unsaturation in order that commercially valuable by-products may be' obtained. In accordance with the preferred procedure for the reduction of hexavalent chromium to trivalent chromium, the carbon chain is split at the points of unsaturation with the formation of carboxyl groups-from each of the carbon atoms so joined. It is known that as the carbon chain ofmonocarboxylic fatty acids is decreased to acetic acid, the stability of the molecule to oxidation is increased and since long chain fatty acids, such as lauric, myristic, stearic, etc., pass through the process substantially unchanged, it is immaterial from the standpoint of production of monocar boxylic acids where the points of unsaturation occurin the unsaturated fatty acid provided-that acetic acid is the lowest molecular weight acid formed. I have found that in the case of certain unsaturated fatty acids the double bonds are so situated as to yield dibasic acids having less than four carbon atoms, in which case such dibasic acids usually undergo further oxidation to carbon dioxide and water. It is therefore desirable to employ a fatty acid capable of yielding a dibasic acid having at least four carbon atoms, although it is to be understood that this type of unsaturated fatty acid is not essential for the reduction of the hexavalent chromium.

The reaction may be carried out by using mixturesoi unsaturated fatty acids which may contain saturated acids. For example, a commercial oleic acid or red oil, consisting essentially of a. mixture of oleicacid (A9 octadecenoic acid), linoleic acid (octadecadien-Q, 12-oic acid) and saturated fatty acids has been found to be efiicacious in reducing hexavalcnt chromium, the reaction yielding azelaic, pelargonic and caproic acids which may be separated in any appropriate manner, hereinafter more fully described. Malonic acid, if formed from the linoleic acid, is not recovered from the organic product and is probably further oxidized to carbon dioxide and water, and any saturated acids present usually pass through the process essentially unchanged.

While the proportions of the ingredients entering into the reaction may be varied throughout a wide range, it is emcacious to maintain proportions of dichromate and unsaturated organic compound which will produce a substantially complete reaction. Although the proportions of water, dichromate and acid in the oxidizing solution may be varied considerably, I have found that a solution having the following composition is satisfactory:

Parts by weight Sodium dichromate dihydrate 100 Water 125 Sulphuric acid (66 Baum) 140 ratios of 1:1 and 1.67:1 of 66 Baum sulphuric acid to sodium dichromate dihydrate. respective- .-ly) without eiiecting a material change in the trend of the reaction. However, when the lower ratio is used the reaction takes place more slow; ly.. "It is advisable to maintain the temperature for the most part below 80 C. in order to prevent a the formation of chromium salts of organic acids.

As an example in carrying out my process using red oil as the reducing agent, a chromium solu tion, prepared as above described, is slowly added to the red oil, the mixture being agitated during the addition and the agitation being continued until the reaction has been substantially completed. The reaction mixture may be maintained at any temperature between 15 C. and its boiling point, although at low temperatures the reaction takes place slowly, whereas at high temperatures undesirable side reactions may occur. A temperature of from 80 to 100C. is a satisfactory mean between the two extremes.

After having carried the reaction to substantial completion, the agitation is discontinued and the reaction product is permitted to separate out. The organic products form a supernatant layer from which the reduced chromium solution may be readily separated and withdrawn for subse-' quent processing. As the organic layer contains small amounts of reduced chromium compounds after separation, it is washed by agitation with an aqueous sulphuric acid solution preferably at a temperature of 80 to 100 0., since the chromium compounds are more readily extracted at this temperature range. If the concentration of sulphuric acid is too low, the solution has a tendencv to extract the water-soluble organic ingredients of the organic layer, but if the concentration of acid is too high the solution will dissolve in or react with the organic products. A 25% to 30% aqueous sulphuric acid solution has been found to produce satisfactory results. The

acid wash is run ofi and if desired may be used in preparing a fresh chromium solution for subsequent treatment.

After the acid wash the organic layer may be washed once or twice with an approximately 30% aqueoussodium sulphate solution, after which the organic products are ready for separation and purification treatment by any of the methods presently to be described.

Method A hydrocarbons such as pentane, hexane, commercial cuts boiling below 400 F., etc., are satisfactory. The precipitated azelaic acid is filtered oil and dried, and the filtrate is subjected to suitable distillation to recover the caproic and pelargonic acids. The organic residue, having a boiling point higher than pelargonic acid, consists essentially of unreacted oleic acid and various saturated acids originally present in the red oil.

Method B The organic product may be subjected to a suitable fractional distillation to remove the caproic and pelargonic acids which are carried over in the first two fractions, and thus recovered directly, the azelaic acid and higher boiling point ingredients being carried over in the next fraction. The distillation residue is saponified with hot caustic, filtered, acidified, separated from the aqueous layer and then distilled, the distillate being added to the previous azelaic acid distillate, this treatment being necessary in order to break down the higher boiling anhydrides of azelaic acid formed during the first distillation and to remove possible residual chromium compounds. .The combined azelaic acid distillates are treated with a petroleum hydrocarbon as in Method A--the oleic and saturated acids being recovered from the filtrate by distilling off the petroleum solvent.

Method c The organic product is first subjected to a suitable fractional distillation to remove the caproic and pelargonic acids, as in Method B, and the distillation residue is extracted with boiling water, the extract being separated from the organic layer. cooled and filtered to remove the precipitated azelaic acid. The extraction is then repeated, using the mother liquor from the first extraction. Four such extractions are usually' suflicient to remove substantially all the azelaic acid when the ratio of water or mother liquor to the distillation residue is of the order of 15:1 (by weightl. When a lower ratio is used a greater number of extractions are usually necessary, and conversely, in the case of a higher ratio of water to-organic product, excepting that-with very low ratios the residue may become soluble or dispersed in the hot concentrated aqueous azelaic acid solution and prevent suitable separation.

The reduced chromium solution separated from the original reaction product usually comprises a chromium sulphate which may or may not conform to the formula Cr(OH)S O4 (corresponding to a. basicity'of 33 depending upon the aciddichromate ratio of the original solution prior to reduction. As chromium sulphate solutions having appreciably lower basicitythan corresponds to the formula Cr(OH)SO4 have slight 5 tanning capacity, it is necessary either to carry out the reduction reaction using an acid-dichromate ratio suificiently low to yield a tan of the composition Cr(OH)SO4, or an adjustment of the reduced chromium solution must be 'madein order to convert the more acid product to Cr(OH) S04. a

In the first case the desired results may be attained by employing a dichromate solution containing three moles of sulphuric acid to one mole of dichromate, the reduction reaction proceeding as follows:

But where-the reduction reaction is carried out using a dichromate solution containing four moles of acid to one mole of dichromate, as is preferred, there is produced a chromic sulphate, as shown by the following equation:

' reducing 3(r2(S0i)3+Nl2Cr2O7+4HzO--(m 8(ir(OH)SO +NB2SO4 The Cr(OH) S04 thus produced may be recovered from the final reaction product in accordance with any of the procedures well known in the art.

It has been found from practical experience in. the tanning of skins with basic chromic sulphate that superior leather is obtained from basic chromic sulphate made by the reduction of hexavalent chromium salts, in accordance with the present invention. as compared with leathers made by the use of basic chromic sulphate obtained from the reduction of hexavalent chromium salts withinorganic agents such as sulphur dioxide. This superior property is attributed to relatively small quantities of oxidized organic materials which remain in the final product,

It is to be understood that this disclosure is for the purpose of illustration only, and that various changes and modifications may be made without departing from the spirit and scope of the invention as set forth in the appended claims.

I claim:

1. The process of preparing a tanning material, which comprises treating a hexavalent chromium compound .with an acid resistant to oxidation and of sufiicient strength to liberate free chromic acid and keep the trivalent chromium in solution, and treating the resulting product with an unsaturated organic compound to produce a reac-, tion product containing a trivalent chromium salt.

2. The process of preparing a tanning material, which comprises treating a hexavalent chromium compound with an acid resistant to oxidation and of suflicient strength to liberate free chromic acid and keep the trivalent chromium in solution, and treating the resulting product with an organic compound having an olefinic double bond to produce a reaction product containing a trivalent chromium salt.

3. The process of preparing a tanning material, which comprises treating a hexavalent chromium compound with an acid resistant to oxidation and of sufilcient strength to liberate'free chromic acid and keep the trivalent chromium in solution, and treating the resulting product with an unsaturated fatty acid to produce a reaction product containing a trivalent chromium salt.

4. The process of preparing a tanning material, which comprises treating a hexavalent chromium compound with an acid resistant. to oxidation and of sufficient strength to liberate free chromic acid and keep the trivalent chromium in solution, and treating the resulting product with oleic acid to produce a reaction product containing a triValent chromium salt. g

5. The process ofpreparing a tanning material, which comprises treating a hexavalent chromium compound with an acid, resistant to oxidation and of suflicient strength to liberate free chromic acid and keep the trivalent chromium in solution, and treating the resulting product with red oil to produce a reaction product containing a trivalent chromium salt.

6. The process of preparing a tanning material, which comprises treating an alkali metal dl-. chromate with an acid resistant to oxidation and of suflicient strength to liberate free chromic acid and keep the trivalent chromium in solution,

and treating the resulting product with an unsaturated organic compound to produce a reaction product containing a trivalent chromium salt.

7. The process of preparing a tanning material, which comprises treating an alkali metal dichromate with an acid resistant to oxidation and of sufficient strength to liberate free chromic acid and keep the trivalent chromium in solution, and treating the resulting product with an unsaturated fatty acid to produce a reaction product containing a trivalent chromium salt,

8. The process of preparing a tanning material, which comprises treating an alkali metal dichromate with an acid resistant to oxidation and of sufficient strength to liberate free chromic acid and keep the trivalent chromium in solution, and treating the resulting product with oleic acid to produce a reaction product containing a trivalent chromium salt.

9. The process of preparing a tanning material,

. which comprises treating an alkali metal dichromate with an acid resistant to oxidation and of sufiicient strength to liberate free chromic acid and keep the trivalent chromium in solution, and treating the resulting product with red oil top roduce a reaction product containing a trivalent chromium salt.

10. The process of preparing a tanning material and fatty acid by-products, which comprises reacting four moles of sulphuric acid with one mole of a dichromate, treating the reaction product with an unsaturated fatty acid to produce a chromium sulphate, reacting the resulting product with one-third mole of a dichromate and a reducing agent, thereby to produce a basic chromium sulphate and organic acid by-products, and segregating the basic chromium sulphate from said organic acid by-products.

11. The process of preparing a tanning material and fatty acid by-products, which comprises reacting four moles of sulphuric acid with one mole of an alkali metal dichromate, treating the reaction product with oleic acid to produce a chromium sulphate, reacting the resulting product with one-third mole of an alkali metal dichromate and a reducing agent, thereby to produce a basic chromium sulphate and organic acid by-products, and segregating th basi ch mium sulphate from said organic acid by-products.

12. The herein described process which comprises reacting a hexavalent chromium compound with an unsaturated organic compound in the presence of sufiicient acid to liberate free chromic acid and maintain in solution trivalent chromium, thereby to produce a reaction product comprising a trivalent chromium compound and a mixture of organic acids, segregating the chromium compound and organicacids, and treating said organic acids to recover the constituents thereof.

13. The herein described process which com- 20 prises reacting a hexavalent chromium compound with an unsaturated fatty acid in the presence of suiiicient acid to liberate free chromic acid and maintain in solution trivalent chromium, thereby to produce a reaction product compris- 5 ing a trivalent chromium compound and a mixture of organic acids, segregating the chromium compound and organic acids, and treating said organic acids to recover the constituents thereof.

14. The herein described process which comprises reacting a hexavalent chromium compound and an organic compound having the formula RCH=CH(CH:) :COOH

. wherein It may be hydrogen, a hydrocarbon radical, or a substituted hydrocarbon radical, and :2:

15. The herein describedprocess which comprises reacting a hexavalent chromium compound and oleic acid in the presence of sumcient acid -to liberate free chromic acid and maintain in solution trivalent'chromium, thereby to produce a reaction product comprising a trivalent chromium compound and a mixture of organic acids, segregating the chromium compound and organic acids, and treating said organic acids to recover the constituents thereof.

16. The herein described process which comprises reacting a.hexavalent chromium compound and red oil in the presence of sufficient acid to liberate free chromic acid and maintain in solution trivalent'chromium, thereby to produce a reaction product comprising a trivalent chromium compound and a mixture of organic acids, segregating the chromium compound and organic acids, and treating said organic acids to recover the constituents thereof.

17. A process of producing simultaneously a tanning liquor and useful organic products by treating an acidified hexavalent chromium solution with an unsaturated organic material to produce simultaneously a tanning liquor by reducing the chromium, and useful organic products by the oxidation of the said unsaturated organic material.

18. A tanning composition comprising a basic chromium sulphate containing a small amount of an oxidation reaction product of an unsaturated organic compound.

19. A tanning composition comprising a basic chromium sulphate containing a small amount of an oxidation reaction product of an unsaturated fatty acid.

20. A tanningcomposition comprising a basic chromium sulphate containing a small amount LAURENCE R. B. HERVEY.

of an oxidation reaction product of oleic acid. 4o 

